Goddard Space Flight CenterGreenbelt, Maryland 20771

FS-2002-9-047-GSFC

The Geoscience Laser Altimeter System (GLAS) on the Ice, Cloud, and land Elevation Satellite (ICESat) spacecraft immediately followingits initial mechanical integration on June 18th, 2002. Note that ICESat’s solar arrays have not yet been attached. Left – Gordon Casto,NASA/GSFC. Right – John Bishop, Mantech. Courtesy of Ball Aerospace & Technologies Corp.

“Possible changes in the mass balance of the Antarcticand Greenland ice sheets are fundamental gaps in ourunderstanding and are crucial to the quantificationand refinement of sea-level forecasts.”

—Sea-Level Change report, National ResearchCouncil (1990)

Ice, Cloud and land Elevation Satellite (ICESat)Are the ice sheets that still blanket the Earth’s poles growing or shrinking? Will global sea level rise orfall? NASA’s Earth Science Enterprise (ESE) has developed the ICESat mission to provide answers tothese and other questions — to help fulfill NASA’s mission to understand and protect our home planet.The primary goal of ICESat is to quantify ice sheet mass balance and understand how changes in theEarth's atmosphere and climate affect the polar ice masses and global sea level. ICESat will also measureglobal distributions of clouds and aerosols for studies of their effects on atmospheric processes andglobal change, as well as land topography, sea ice, and vegetation cover.

Ice sheets are complex and dynamic elements of our climate system. Their evolution has strongly influenced sea level in the past and currently influences the global sea level rise that threatens ourcoasts. Ice streams that speed up, slow down, and change course illustrate their dynamic nature.Atmospheric factors cause snowfall to vary in space and time across their surfaces. In Antarctica, smallice shelves continue to retreat along the Antarctic Peninsula, and large icebergs are released from thelargest ice shelves. In Greenland, the ice sheet margins are thinning and the inland parts of the ice sheetappear to be thickening. Surface meltwater seeps into the ice sheets and accelerates their flow. Some ofthe factors controlling the mass balance of the ice sheets, and their present and future influences on sealevel, are just beginning to be understood.

The ICESat mission, part of NASA’s Earth Observing System (EOS), is scheduled to launch in December2002. The Geoscience Laser Altimeter System (GLAS) on ICESat will measure ice sheet elevations,changes in elevation through time, height profiles of clouds and aerosols, land elevations and vegetationcover, and approximate sea ice thickness. Future ICESat missions will extend and improve assessmentsfrom the first mission, as well as monitor ongoing changes. Together with other aspects of NASA’s ESEand current and planned EOS satellites, ICESat will enable scientists to study the Earth’s climate and,ultimately, predict how ice sheets and sea level will respond to future climate change.

“In light of…abrupt ice-sheet changes affecting global climate and sea level, enhanced emphasis on ice-sheetcharacterization over time is essential.”

—Abrupt Climate Change report, National Research Council (2002)

MISSION INTRODUCTION AND SCIENTIFIC RATIONALE

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Are The Greenland AndAntarctic Ice Sheets GrowingOr Shrinking?This question lies at the heart of NASA’s rationale for theICESat mission. The Greenland and Antarctic ice sheets are anaverage of 2.4 km (7900 ft) thick, cover 10 percent of the Earth’sland area, and contain 77 percent of the Earth’s fresh water (33million km3 or 8 million mi3). The Antarctic ice sheet has 10 timesmore ice than Greenland because of its greater area and average icethickness. If their collective stored water volume were released intothe ocean, global sea level would rise by about 80 m (260 ft). Asmall change of only 0.1% (2.4 m or 7.9 ft) in the average thick-ness of the ice sheets would cause a change in global sea levelof 8.3 cm (3.3 in). Their vast size and inhospitable environ-ment make the ice sheets impossible to monitor completelyexcept via satellite.

Presently, new ice accumulates on the ice sheets at about 17cm (7 in) per year averaged over Antarctica and 28 cm (11 in)per year averaged over Greenland. Although most of theAntarctic ice sheet has net ice accumulation, about 9% ofGreenland has a net ice loss of 1.4 m due to melting in the summer.Together, the volume of water required for this annual accumulationof ice is equal to the removal of 0.8 cm (0.3 in) of water from the entire surface of the Earth’s oceans. Approximately the same amount of water is returnedto the oceans by melting of ice and snow, by ice flow into the floating ice shelves, and the direct discharge of icebergs. However, the exact amounts being gained or lost vary significantly from region toregion as well as through time. The difference between the total mass input and total mass output(called the mass balance) is important because it might be as much as ± 20% of the mass input, which isequivalent to a sea level rise or fall of 0.16 cm (0.06 in) per year.

ICESat is designed to detect changes in ice sheet surface elevation as small as 1.5 cm (0.6 in) per yearover areas of 100 km by 100 km (62 mi by 62 mi). Changes in ice sheet thickness are calculated from elevation changes by correcting for small vertical motions of the underlying bedrock. Ice thicknesschanges will enable scientists to assess the ice behavior within individual ice drainage basins and majoroutlet glaciers, as well as the entire ice sheets. Detection of average thickness changes as small as 0.3 cm (0.12 in) per year over Greenland and Antarctica will reduce the uncertainty in their contribution to sealevel change to ± 0.1 cm (0.04 in) every 10 years. Elevation time-series constructed from ICESat’s contin-uous observations throughout its 3 to 5 year mission will detect seasonal and interannual changes in themass balance, caused by short-term changes in ice accumulation and surface melting, as well as thelong-term trends in the net balance between the surface processes and the ice flow.

Courtesy

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Rober

t Sim

mon

How Fast Is Sea Level Rising?Fifteen thousand years ago, vast ice sheets covered much ofNorth America and parts of Eurasia. In Antarctica, the icesheet reached to the edge of the continental shelf, as muchas 200 km (125 mi) farther out to sea than today. As the climate warmed during the end of the last Ice Age, much ofthe Earth’s ice cover melted. Global sea level rose rapidlyby almost 100 m (330 ft) between 14,000 and 6,000 yearsago. Although the average rise was about 1.25 cm per year(0.5 in), the total rise caused dramatic changes to coastlinesaround the world.

The Antarctic and Greenland ice sheets, which are the mostsignificant remnants of that period, are currently reactingto present and past climate changes. Global sea level isbelieved to be rising about 2 cm (0.8 in) every 10 years, lessrapidly than at the end of the ice age, but still significant.About 25% of the present rise is caused by thermal expan-sion as the oceans warm, and about 25% by the melting ofsmall glaciers around the world. The remainder could bedue to ice loss from Greenland and Antarctica, but ouruncertainty of their actual contributions (plus or minus) is as large as the total rise in recent decades. Sealevel rise is also affected by human activities such as pumping of ground water, filling of reservoirs,deforestation and biomass burning, and draining of wetlands. Although sea level changes are small inany given year, sea level increases over the last 50 to 100 years have seriously impacted low-lyingcoastal regions through shoreline retreat, storm erosion, and coastal flooding. Future changes associatedwith climate warming may cause an even greater threat to our populous coastal environments.

Of particular concern is the “marine” ice sheet in West Antarctica,much of which is grounded on bedrock and sediment 1500 m (4900 ft)below sea level. The three main components are: 1) interior ice that flows relatively slowly (10 m or33 ft per year); 2) the fast-moving ice streamsthat flow a hundred times faster; and 3) thefloating ice shelves through which most of theice enters the ocean. Some researchers believethat if West Antarctica’s ice shelves thinned significantly, they would no longer restrain therate of ice discharge from the ice streams. ICESat’smulti-year elevation-change data, combined with other remote sensing data, as well as atmospheric, oceanic, and ice flow models,should enable more accurate predictions of ice-sheet changes.

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Examples of coastal erosion along theeastern seaboard of the United Statesfollowing storms.

Published data on sea level rise with time (Fairbanks, 1989).Current sea level rise is about 2 cm per decade.

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Will The Ice Sheets ThinOr Thicken In A WarmerClimate?Increased warmth will melt more ice near theedges of the ice sheets. However, a warmeratmosphere holds more moisture, leading tomore precipitation over the ice sheets and moreice accumulation. Changes in atmospheric circulation may also change the frequency ofstorms and the transport of moisture onto theice sheets. A warmer climate should alter thearea and thickness of sea ice surrounding theice sheets, and thereby shorten or lengthen themoisture pathway for snowfall. Other impor-tant processes include redistribution of newsnow by wind and sublimation from the icesheet surface.

Atmospheric interactions with the ice sheetshave immediate effects on the ice sheet massbalance at the surface and therefore cause year-to-year effects on both the amount of waterstored and global sea level. In contrast, the rateof ice flow tends to respond slowly to climate change, over centuries or longer, as temperature and othereffects slowly propagate deep into the ice. However, the recent discovery of accelerations in ice flowduring periods of increased summer melting in Greenland demonstrates a rapid dynamic response ofthe ice to climate change. Similarly in Antarctica, increases in surface and basal melting on ice shelvescauses them to thin, which may lead to an acceleration of grounded ice discharge from the interior.

At present, we do not know which effect will have more impact on the overall ice sheet mass - increasedmelting and ice thinning at the edges or increased snowfall and ice accumulation over the entire icesheet surface. The net change in mass input (snowfall) minus output (melting and icebergs) in the com-ing decades could be as much as -10 % to +10 % for each degree Celsius of climate warming, resultingin a change of -0.8 to +0.8 cm (-0.3 to +0.3 in) in global sea level per decade for each 1° C of temperaturechange. Over centuries, the interaction of climate with the ice flow could amplify the changes.

ICESat will measure seasonal and interannual variations in ice sheet elevations caused mainly by snow-fall and surface melting, as well as enable estimates of the long-term trend in the overall mass balance.Scientists will use these data to answer questions such as which variables (e.g. temperature or precipita-tion) exert control on the ice sheet mass balance, and whether the observed ice changes are caused byrecent or long-term changes in climate. These studies combined with ice models will be used to predictfuture changes in ice volume that would be linked to climate change.

Ice Flow

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Illustration of key climate variables influencing ice sheet mass balance.Graphic by Deborah McLean.

Surface melt water ponds on the western flank of the Greenland ice sheet.Courtesy of Konrad Steffen.

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How Will ICESat Measure Earth’s Ice, Clouds, Oceans,Land, and Vegetation?The GLAS instrument on ICESat will determine the distance from the satellite to the Earth’s surface andto intervening clouds and aerosols. It will do this by precisely measuring the time it takes for a shortpulse of laser light to travel to the reflecting object and return to the satellite. Although surveyors rou-tinely use laser methods, the challenge for ICESat is to perform the measurement 40 times a secondfrom a platform moving 26,000 km (16,000 mi) per hour. In addition, ICESat will be 600 km above theEarth and the precise locations of the satellite in space and the laser beam on the surface below must bedetermined at the same time.

The GLAS instrument on ICESat will measure precisely how long it takes for photons from a laser topass through the atmosphere, reflect off the surface or clouds, return through the atmosphere, collect inthe GLAS telescope, and trigger photon detectors. After halving the total travel time and applying corrections for the speed of light through the atmosphere, the distance from ICESat to the laser footprinton Earth’s surface will be known. When each pulse is fired, ICESat will collect data for calculating exactly where it is in space using GPS (Global Positioning System) receivers. The angle at which thelaser beam points relative to stars and the center of the Earth will be measured precisely with a star-tracking camera that is integral to GLAS. The data on the distance to the laser footprint on the surface,the position of the satellite in space, and the pointing of the laser are all combined to calculate the elevation and position of each point measurement on the Earth.

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Schematic illustration of the GLAS instrument making measurement from ICESat while orbiting the Earth.Graphic by Deborah McLean.

OPERATIONAL OVERVIEW

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GLAS measures continuously along ground tracks defined by the sequence of laser spots as ICESatorbits the Earth. The GLAS laser pulses are emitted at a rate of 40 per second from the Earth-facing(nadir) side of ICESat. This produces a series of approximately 70 m (230 ft) diameter spots on the surface that are separated by nearly 170 m (560 ft) along track. These tracks will be repeated every 8days during the initial calibration-validation phase of the mission and every 183 days during the mainportion of ICESat’s multi-year mission. Points on the Earth where these ground tracks intersect arecalled crossovers. Crossovers increase in areal density near the North and South Poles. Over the icesheets, the ICESat spacecraft will be controlled to point the laser beam precisely toward the same repeatground tracks to within ± 35 m. Therefore, elevation changes can be analyzed along repeat groundtracks as well as at orbital crossover points. In addition, ICESat has the ability to point GLAS off-nadirto repeatedly measure areas of interest such as an erupting volcano or a collapsing ice shelf.

ICESat will also provide detailed information on the global distribution of clouds and aerosols. To dothis, GLAS emits laser energy at both 1064 nm and 532 nm, as this allows co-located elevation andatmospheric data to be obtained simultaneously. This will aid precise determination of the distancebetween ICESat and the Earth, as it is important to know if the laser pulses have traveled through cloudand aerosol layers. If present, these phenomena will diffuse the laser energy and extend the distance thelaser light photons travel by scattering.

Although ICESat’s data on ice sheet surface elevations isof primary importance, land and ocean elevationdata will be obtained around the globe alongICESat’s ground tracks. Data on elevationchanges of the world’s oceans and ice-free landforms is expected to be ofgreat interest to the broader scientific community. The recentlylaunched GRACE satellite willenhance the ICESat mission bymapping the Earth’s gravita-tional field in unprecedenteddetail. The GRACE data, inconjunction with ICESat resultswill enable a greater under-standing of any changes in thedistribution of snow, ice, andwater mass around the globe.

Illustration of ICESat’s 8 day repeat groundtrack relative to the continental area ofAntarctica. Note the convergence around the SouthPole of the ground tracks. ICESat’s orbit wasdesigned to maximize coverage over the polar ice sheets.The pattern is similar over the northern polar region.Courtesy of Bob Schutz.

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Rapid Thinning Of Greenland’s Coastal IceAfter Antarctica, Greenland’s ice sheet contains the second largest mass of frozen fresh water on Earth.The island of Greenland covers 2,175,000 square kilometers (840,000 square miles) and 85 percent is covered by ice up to 3370 m (11,050 ft) thick. With its southern tip protruding into temperate latitudes, itis more sensitive than Antarctica to climate changes experienced in the major population centers ofEurope and North America. In contrast to the Antarctic ice sheet, for which most of the surface remainsbelow the freezing point even in summer, 85% of the Greenland ice sheet has melting on the surfaceduring summer. In the zone of net ablation below about 1200 m, all the winter snow plus several metersof ice melts each year. Therefore, the mass balance of the Greenland ice sheet is very sensitive to changesin temperature and melting, as well as to changes in accumulating snowfall.

A NASA study of Greenland’s ice sheet during the 1990s revealed rapid thinning at its margins, but alsosome thickening across portions of the interior. A NASA aircraft flew a network of flight lines with alaser altimeter and global positioning satellite receivers in 1993 and 1994 and re-surveyed the lines in1998 and 1999. From these data, scientists reported that the ice sheet has thinned by more than 1 m (3 ft)per year on many coastal outlet glaciers and by as much as 9 m (30 ft) per year in a few areas. Scientistsestimated a net loss of approximately 50 cubic kilometers (12 cubic miles) of ice per year from the entireice sheet during the 1990s. This volume is sufficient to raise global sea level by 0.13 mm (0.005 inches)per year, or about seven percent of the observed rate of rise in recent decades.

Explaining these observations and the ice andclimate processes causing them is the subject ofongoing study by the Program for ArcticRegional Climate Assessment (PARCA). Thisjoint effort by NASA and affiliated scientistsconsists of combined remote sensing, fieldwork,and modeling studies on the Greenland IceSheet. ICESat will contribute to PARCA byobtaining elevation data across Greenlandthroughout the year in a more consistent fashion than could ever be obtained by the pioneering airborne effort.

Results from a multi-year airborne laser altimetry study ofthe Greenland ice sheet. The map of observed elevationchanges indicates significant variations in the changesfrom place to place and the complex behavior of the icemass. Blue, green, and gray tones indicate areas of ice lossdue to melting, reduced snowfall, or increased ice dis-charge. The pale-yellow, orange-brown, and red colorsover much of the interior of the ice sheet indicate areaswhere the ice surface is rising. Courtesy of Bill Krabill.

EARTH’S DYNAMIC ICE

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Thinning of Part of the West Antarctic Ice Sheet An additional illustration of ice sheet elevation changes is provided by analysis of 7 years of EuropeanRemote Sensing Satellite (ERS-1 and 2) satellite radar altimetry data (1992-1999) over part of the WestAntarctic ice sheet (see pages 10-11). The large area of green and blue in the upper part of the figure covers much of the ice drainage basin that feeds into two major outlet glaciers (Pine Island Glacier andThwaites Glacier). Elevation decreases there are as large as 30 cm (12 in) per year. The ice thinning islikely to be a continuing response to climate change over manycenturies, and perhaps removal of an ice shelf in front of the glaciers in the past. Elevation increases up to 20 cm (8 in) per yearare indicated (red and orange) on the ridge between the PineIsland/Thwaites drainage basin and the Ross Ice Shelf to thesouthwest (lower right).

The average elevation change for the grounded ice (excluding thefloating ice shelves) over this region is – 4.3 cm (1.7 in) per year,and the estimated average rate of bedrock uplift is 2.5 cm (1 in) peryear. Therefore, the estimated rate of ice thinning is 6.8 cm (2.7 in)per year, which means a mass loss of 73 km3 (17.5 mi3) of ice peryear. The resulting contribution to sea level rise is 0.2 mm (0.008in) per year from this part of West Antarctica. ICESat will improvethe resolution and accuracy of such measurements, especiallynear the coast where steeper slopes limit radar altimetry, andextend the measurement inland to 86° S.

Thinning of the Larsen Ice Shelf – Antarctic PeninsulaAnalysis of the ERS-1 and 2 radar altimeter data on the northerly part of the Larsen Ice Shelf (“LarsenB”) for the period 1992 to 1999 (see pages 9 and 10) showed a lowering of the elevation of the ice shelfsurface by 27 cm (11 in) per year. A 27-cm change in the height of the ice shelf surface above sea levelmeans the total ice thickness changed by about 1.7 m (5.6 ft), because only 16% of the thickness of float-ing ice shelves is above the water line. Therefore over the 7 years, the shelf lost about 12 m (39 ft) of itstotal thickness of about 170 m (560 ft). The breakup the northern parts of the Larsen Ice Shelf and otherice shelves in this region has been associated with significant warming of the Peninsula region in recentdecades. As shown in the figure, the surface elevationof the remaining part of the Larsen Ice Shelf (“C”) hasalso been lowering at a similar rate of 21.7 cm (8.5 in)per year, which implies ice thinning of 1.4 m (4 ft)/year. Although the Larsen C is about 100 m (330 ft)thicker than B was, it may face a similar breakup ifwarming continues.

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Time series of surface elevation change of the Larsen C IceShelf, Antarctic Peninsula. Courtesy of Jay Zwally.

Spatial illustration of surface elevation changeacross a part of West Antarctica from ERS-1 and -2radar altimetry (Zwally et al., 2002).

Larsen B Ice Shelf - Austral Summer 2002As predicted beforehand by scientists, the rapid thinning of the Larsen Ice Shelf led to a series of dra-matic events captured by satellite images as well as witnessed by Argentine field researchers.

January 31, 2002 (top left image) — Melt ponds, with light blue tones, are observed across much of theice shelf in surface depressions associated with crevasses and flow lines. At this time, the Larsen B iceshelf extended from Robertson Island (at the upper left) to the Jason Peninsula (along the lower margin),following a significant retreat between 1993 and 2001 from its “historical” observed extent earlier in thelast century.

February 17, 2002 (top right image) — Small events resulted in a number of icebergs with a variety ofsizes calving from the ice shelf front. Melt ponds were still evident but appear to be diminished inextent. The melt ponds became less obvious as the ice shelf began to break apart and water drainedthrough the ice shelf to the ocean below.

March 5, 2002 (bottom left image) — Collapse of the majority of the Larsen B ice shelf. Due to wind andtidal forces, new icebergs now choke the embayment from the ice shelf front to the end of RobertsonIsland. The light blue color is the result of the exposure of glacial ice as the ice fragments rotate from avertical to a horizontal position in the open water of the bay.

March 17, 2002 (bottom right image) — Disintegration of the ice shelf margins continues with dispersalof the ice shelf fragments due to wind and tidal forces. The area lost is about 3250 km2 (1250 mi2) orapproximately the area of Rhode Island, with a weight estimated at 720,000,000,000 tons.

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January 31, 2002

March 17, 2002

Collapse of the Larsen B iceshelf captured by a sequenceof satellite imagery. SeeScambos et al., J. Glaciology,2000, for a history and mecha-nism of ice shelf collapses inAntarctica. The data were col-lected by the ModerateResolution ImagingSpectrometer (MODIS) onNASA’s Terra satellite.Courtesy of Ted Scambos.

February 17, 2002

March 5, 2002

Antarctica, thefrozen continent, isa landscape that haslong challenged andintrigued humanity. Thisimage from Radarsat showsspatial details with claritynever previously achieved bysatellite observations. Thisimage serves to provide a contextfor ICESat’s measurements of surfaceelevation change. To illustrate the scaleof the ice mass that may influence globalsea level, keep in mind that theAntarctic continent is approximatelytwo thirds the size of North Americaor equivalent to the combined areasof the United States and Mexico.From the Antarctic Peninsula, withits fringing ice shelves that are progres-sively collapsing, to West Antarctica, with itsmarine-based (below sea level) ice sheet andhuge ice shelves, to East Antarctica with its greatthickness (> 4,000 m or 12,000 ft), this continent’ssurface features are revealed in detail.

This Radarsat Antarctic Mapping Mission mosaicillustrates the complex surface that ICESat must meas-ure throughout its mission. However, the image illustratesonly the areal or 2-dimensional character of this ice sheet; ICESatwill provide the needed 3rd dimension, surface elevation as well as its changethrough time. Note the huge Ronne-Filchner and Ross Ice Shelves that have produced huge icebergssince this mosaic was obtained. The twisted patterns of ice draining from the interior of the ice sheet outto the ice shelves are extraordinary. Although not distinct at this resolution, ice streams can be identifiedby the bright radar returns from their crevassed (fractured) margins that reflect radar energy more effi-ciently than surrounding unfractured interior ice. These ice streams are vast rivers of ice that flow up to100 times faster than the ice they channel through, with speeds up to 1 km (3000 ft) per year. Some icestreams in East Antarctica extend almost 800 km (500 mi). They have changed their velocity and flowpatterns in response to factors that are just now beginning to be understood through field research supported by intermittent satellite data. Ice streams form the most dynamic parts of the Antarctic ice

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RADARSAT “Snapshot” Of An Ice Sheet

Antarctic Peninsula

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est Antarctica

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sheet, and scientists believe that they arequite susceptible to rapid change.

ICESat surface elevation time series datawill enhance our understanding ofdynamic ice. Because of their relativelyfast flow, ice streams are responsiblefor transporting a large percentage ofthe snow that falls on the continent’s interior to the ocean. ICESat will

monitor the elevation details of thisdynamic surface for 3 to 5 years with

far greater accuracy thanRadarsat. Images from sensors

on other satellites will providecontext for this mission’s

elevation change data.

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Radarsat can collect thesespatial data day and night,through cloudy weather orclear. It was this capabilitythat enabled Antarctica to bemapped in just 18 days,compared to the last satel-lite map that requiredimages from five differentsatellites spanning a 13-year

period from 1980 to 1994.Following a precise orbital

maneuver, Radarsat acquired complete coverage of the continent

as part of the first AntarcticMapping Mission (AMM-1). A

follow-up mission (MAM) took placeduring 2000. The Radarsat Antarctic

Mapping Project (RAMP) is a joint effort ofNASA and the Canadian Space Agency (CSA).

The Byrd Polar Research Center, Vexcel Corp.,the Alaska SAR Facility, and the Jet Propulsion

Laboratory conducted the project collaboratively,with funding from NASA’s Pathfinder and

Cryospheric Programs. The project also received valu-able assistance from the National Science Foundation,

the Environmental Research Institute of Michigan, andthe National Imagery and Mapping Agency. For more

information on Radarsat data and this mosaic, contact theNational Snow and Ice Data Center (NSIDC).

Murdo Station

Amery Ice Shelf

East Antarctica

cticM

ountains

How Do Clouds and Aerosols Affect Climate?The distribution of atmospheric clouds and aerosolsis one of the most important factors in global climate.Clouds can cool the Earth’s surface by reflectingsolar radiation or warm the surface by trapping itsradiated heat. Being typically more reflective thanthe Earth’s surface, low clouds primarily reflect solarradiation and cause relative cooling. High, thinclouds are usually less effective at reflecting solarradiation but effectively trap outgoing infrared heatradiation. Accurate knowledge of the type andheight of clouds is thus very important for climatestudies. Like clouds, aerosols tend to cool the Earth’ssurface and heat the atmosphere by scattering andabsorbing solar radiation. In general terms, aerosolsare distinguished from clouds by existing at humiditylevels below saturation and by a particle size which istypically 10 to 100 times smaller than cloud droplets.

The laser profiling measurements from GLAS are a fundamentally new way to study the atmospherefrom space. To better understand climate, scientists need to distinguish the multiple cloud and aerosollayers that typically exist in the atmosphere. Other satellite remote-sensing techniques currently in useare limited to passive observations, i.e. the sensor images the Earth at a given wavelength but views allatmospheric layers simultaneously. Another issue is that such passive instruments cannot measure theheight of layers sufficiently to fullyunderstand the role of clouds in globalclimate change. The GLAS instrumenton ICESat will enable the accurate,multi-year height profiling of atmos-pheric cloud and aerosol layers directlyfrom space for the first time.

GLAS cloud and aerosol measurementswill also improve studies of other atmos-pheric phenomena unique to the polarregions. Polar stratospheric clouds thatform during periods of depleted atmos-pheric ozone can be easily detected fromGLAS measurements. Polar “diamonddust”, which refers to near-surface suspended ice crystals from an appar-ently non-precipitating sky, could also

Illustration of varying cloud optical thicknesses from thehighest thinnest cloud (upper right) to the dense low-lyingclouds (lower left). Courtesy of Art Rangno.

Active profiling of the atmosphere by GLAS (model data shown here)allows individual cloud and aerosol layers to be seen separately, whereastraditional satellite imaging techniques combine the radiation from alllayers into one measurement. Thicker clouds with higher optical densities(brighter colors) are contrasted with thin clouds, aerosols, and clear skies(progressively darker colors). Courtesy of Jim Spinhirne.

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CLOUDS, AEROSOLS, LAND ELEVATION, AND VEGETATION

be detected by GLAS. Finally, GLAS will overcome an enduring limitation of high-latitude research, the inaccessible and uninhabitable nature of much of the polar regions, and make data available from regions that have previously not been well studied.

Clouds play an especially critical role in the climate of the polarregions. The polar temperature balance is dependent on clouds preventing heat loss to space. Passive observations of clouds arecan be inaccurate in these areas because the visible and infraredradiative signatures of clouds are hard to distinguish from that ofthe bright, cold snow and ice covered surfaces below them. Imagetechniques to distinguish between snow-covered surfaces and typical cirrus clouds in these regions are complex, and require considerable data analysis. The GLAS instrument will enableunambiguous detection of all but the thinnest atmospheric layersand this will permit the development of consistent climatologiesof the Arctic and Antarctic. GLAS measurements will alsoimprove studies of polar stratospheric clouds that facilitate depleted atmospheric ozone.

Understanding the role of hazeaerosols is also an issue for climatechange researchers. Atmosphericaerosols globally impact radiativetransfer of energy and also influ-ences cloud formation. In addition,the fertilization for biological activity of large parts of the oceanand parts of the tropical rain forestoccur by aerosol transport.Significantly, aerosol distributionand characteristics are highly vari-able and complex. Aerosols aretransported around the world bywinds that can change dramaticallywith altitude, but no existingobservation can reliably give theheight of aerosol layers. The GLASaerosol profiles will be a uniqueand critical contribution to earthscience and global change research.

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Image of clouds and sea ice around north-ern Greenland from NASA’s Aqua MODISsensor. The 1 km resolution image wastaken July 13, 2002. Courtesy of the AquaMODIS Science Team.

A true-color image acquired May 4, 2001, by the Sea-viewing Wide Field-of-view Sensor reveals a large plume of aerosols blowing eastward over theNorth Atlantic Ocean. Courtesy of the SeaWiFS Project.

Measuring Earth’s Land Surface And VegetationThe topography and vegetation cover of the Earth’s land surface form a complex mosaic that is theproduct of a diverse set of solid Earth, glacial, hydrologic, ecological, atmospheric, anthropogenic, andother processes. The landscape we see today is the cumulative result of the interaction of those processes through time. Measurement of landscape properties, including elevation, slope, roughness,and vegetation height and density, is a necessary step toward understanding the interplay betweenformative processes and thus toward more accurate modeling of future changes. Knowledge of theseproperties and their changes with time is important for resource management, land use, infrastructuredevelopment, navigation, and forecasting the occurrence and impact of natural hazards such as volcaniceruptions, landslides, floods, and wild fires.

The ICESat profiles will provide a global sampling of the elevation of the Earth’s land surface withunprecedented accuracy. This globally-consistent grid of high-accuracy elevation data will be used as areference framework to evaluate and improve the accuracy of topographic maps acquired by other air-borne and space-based methods such as conventional stereo-photogrammetry and radar interferometry.In particular, ICESat profiles will be combined with the near-global mapping accomplished by theShuttle Radar Topography Mission to greatly improve our knowledge of the Earth’s topography. Also,using ICESat’s consistent global framework, topographic maps acquired through time using a variety ofmethods can be better co-registered, enabling long-term observations of topographic changes.

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Dramatic topographic and vegetation cover change can be causedby natural hazards. This is illustrated here by two views of Mt. St.Helens, Washington, from Spirit Lake before (upper) and after(lower) the May 18, 1980 eruption. The upper picture is courtesy ofJim Nieland and the lower picture is courtesy of the USGSCascades Volcano Observatory. The satellite view of Mt. St. Helens(right image) shows the ejected debris field outside the crater and iscourtesy of the Landsat-7 Project.

ICESat profiles repeated through time across dynamic landforms will also enable direct observation oftopographic change. By pointing the ICESat spacecraft off nadir, features can be targeted for profilingevery 12 days on average near the equator, and even more frequently at higher latitudes. This targetingcapability will be used to monitor selected phenomena such as volcanic eruptions, changes in river andlake levels, seasonal snow-pack dynamics, glacier surges and retreats, soil erosion, and migration ofdesert sand sheets. The global access of ICESat’s elevation profiling will add the height dimension toimages of the dynamic Earth acquired by other orbital remote sensing instruments.

In addition to acquiring elevation data, ICESat’s measurement of the laser pulse return shape providesunique information about the height distribution of the surface features within each laser footprint. Inareas lacking vegetation cover, this is a measure of relief (ground slope and roughness), an indication ofthe intensity of geomorphic processes. In vegetated areas of low relief, the elevation of the ground andthe height and densityof the vegetation covercan be inferred fromthe return pulse. Thevegetation observationsenable estimation ofabove-ground biomassand its loss due todeforestation, animportant componentof the carbon cycle.

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Photograph of Yosemite Valley,California, showing complexlandscape elements. Courtesy ofDavid Harding.

Photograph of the Brazilianrain forest canopy. Courtesyof the Large Scale Biosphere-Atmosphere Experiment inAmazonia.

What Is The Geoscience LaserAltimeter System?The Geoscience Laser Altimeter System (GLAS) is a next-generation space lidar. It is the sole science payload forNASA’s ICESat Mission. The GLAS design combines a 15cm (6 in) precision surface lidar with a sensitive dualwavelength cloud and aerosol lidar. GLAS operates withinfrared and visible laser light pulses at 532 nm and 1064nm wavelengths at eye-safe signal levels. These laser lightpulses illuminate the Earth and will enable GLAS to measurethe surface elevation of the polar ice sheets accurately, establisha network of height data on the Earth’s land topography, andprofile the vertical distribution of clouds and aerosols on aglobal scale. GLAS is integrated onto the ICESat spacecraftbuilt by Ball Aerospace. ICESat will be launched into a near-polar orbit that is slightly inclined relative to the equator, sothat it will cover the Earth from 86° N to 86° S.

GLAS will measure the vertical distance from orbit to theEarth’s surface 40 times a second with pulses from a ND:YAGlaser. Each GLAS laser pulse at 1064 nm can yield a single distance measurement as well as other information about thecharacter of the Earth’s surface. On Earth, the laser footprintshave a diameter of approximately 70 m (230 ft) and 170 m (560ft) center-to-center spacing. The GLAS receiver uses a 1 m (3 ft)diameter telescope to collect the reflected 1064 nm laser lightand a detector that precisely times the outgoing and reflectedlaser pulses. A digitizer records each transmitted and reflectedlaser pulse with 1 nano-second (ns) resolution.

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Three views of the GLAS instrument following integration with the ICESat satellite at Ball Aerospace & Technologies Corp. inBoulder, Colorado. Courtesy of Ball Aerospace.

The GLAS instrument following final assem-bly and testing at NASA Goddard SpaceFlight Center. Courtesy of the GLASInstrument Team.

Illustration of the GLAS instrument from thezenith-facing side. Courtesy of the GLASInstrument Team.

INSTRUMENT AND MISSION SPECIFICS

GLAS will also measure the vertical distributions of clouds and aerosols by recording vertical profiles oflaser backscatter at both 1064 nm and 532 nm. A 1064 nm detector will be used to measure the heightand echo pulse shape from thicker clouds. The lidar receiver at 532 nm uses a narrow bandwidth etalonfilter and highly sensitive photon counting detectors. The 532 nm backscatter profiles will be used tomeasure the vertical extent of thinner clouds and the height of the atmospheric boundary layer.

A precision “Blackjack” GPS receiver carried on the ICESat spacecraft will measure GLAS’s location inorbit. Accurate knowledge of the laser’s pointing angle relative to inertial space is needed to minimizeuncertainty when measuring over sloping surfaces such as ice sheet margins. On its “zenith” or star-facing side, GLAS uses a stellar reference system (SRS) to measure the pointing angle of each laser pulserelative to selected bright stars that are effectively static reference points in space. GLAS uses redundanthigh precision star cameras and gyroscopes to determine the orientation of its measurement baseline onthe instrument’s optical bench. Each laser pulse is measured relative to the star camera with a laser reference system (LRS). Analysis of the reflected 1064 nm and 532 nm pulse data, along with the GPSand LRS data, enables final determination of the distance to the reflecting surface, the degree of pulsespreading due to atmospheric conditions, and vertical distribution of any surface vegetation.

Graphical illustration of the GLAS instrument (with itstelescope sun shade and main optical bench removed)showing the 1 m telescope mirror receiving a reflectedlaser pulse (in green) from Earth into its detectors.Courtesy of the GLAS Instrument Team.

Graphical illustration of the GLAS instrument showingLaser-1 firing a laser pulse (in green) toward Earth.Courtesy of the GLAS Instrument Team.

Telescope Sun Shade

Laser 1

12

3

Radiator

Radiator

Main Optical Bench

Stellar Reference SystemInstrument Star Tracker

Altimeter Detectors (2)

Single PhotonCounting Modules (8)

Telescope Bench

1 m Telescope Mirror Secondary Mirror Tower

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Outgoing Laser Light Pulse

Incoming LaserLight Pulse

The ICESat Mission — OverviewThe overall mission is composed of the GLAS instrument, the ICESat spacecraft, the launch vehicle, mission operations, and the science team. Goddard Space Flight Center (GSFC) staff developed theGLAS instrument in partnership with university and aerospace industry personnel. The ICESat space-craft was developed by Ball Aerospace & Technologies Corp. (Ball Aerospace) in Boulder, Colorado. BallAerospace will also support ICESat when it is on orbit. NASA Kennedy Space Center is providing theexpendable Boeing Corporation Delta II launch vehicle. The science team is composed of researchersfrom universities, GSFC staff, and supporting industry personnel. The science team is developing thescience algorithms and is responsible for all science data processing, as well as the generation of sciencedata products.

A Delta II launch vehicle will carry ICESat, as well as a second payload called CHIPSat, into a near-polar low Earth orbit of approximately 600 kilometers altitude. The Delta II will be launched from SpaceLaunch Complex-Two (SLC-2), Western Test Range, Vandenberg Air Force Base, California in mid-December, 2002. While on orbit, ICESat achieves ±10 arcsec pointing accuracy and ~2 arcsec pointingknowledge. To accomplish this, a three-axis stabilized attitude control system composed of two startrackers, four reaction wheels, a hemispherical resonator gyroscope on the GLAS instrument, 15 coarsesun sensors, three magnetometers, and three torque rods is used. Orbital position is derived from aredundant set of NASA JPL “Blackjack” GPS receivers and a global network of ground receivers, provided by the International GPS Service. Additional orbit position information is obtained from theInternational Laser Ranging Service, a network of ground laser stations that will use a laser retroreflec-tor mounted on the nadir (earth-facing) side of ICESat. While on orbit, the spacecraft will communicatewith the Earth Observing System (EOS) Polar Ground Stations (EPGS) four times a day over X-Bandand S-Band radio frequency channels.

Solid RocketMotors Impact

Liftoff

Solid Rocket Motor Burnout (3) t = 1.1 minutesAlt. = 16.3 km (10.1 mi)Vel. = 2340 kph (1450 mph)

Solid Rocket Motor Jettison (3)t = 1.7 minutes Alt. = 31.2 km (19.5 mi) Vel. = 2820 kph (1750 mph)

Main Engine Cut Offt = 4.4 minutes Alt. = 104.7 km (65.1 mi)Vel. = 17430 kph (10830 mph) Second Engine Cut Off I

t = 11.2 minutesAlt. = 188.2 km (117.2 mi)Vel. = 28470 kph (17690 mph)

Dual Payload AttachmentFitting Separationt = 96.7 minutesAlt. = 592.3 km (368.6 mi)Vel. = 27240 kph (16930 mph)

ICESat Separationt = 64.0 minutesAlt. = 590.4 km (367.4 mi)Vel. = 27230 kph (16920 mph)

CHIPSat Separationt = 103.3 minutesAlt. = 585.2 km (364.2 mi)Vel. = 27250 kph (16930 mph)

Depletion Burn:removes Stage 2 fromvicinity of spacecraft,while lowering Stage 2orbit perigee altitude

Second Engine Cut Off II(restarted inflight) t = 59.8 minutes Alt. = 589.1 km (366.6 mi) Vel. = 27230 kph (16920 mph)

I

C

KEY (nominal values) t = Time (since launch) Alt. = Altitude Vel. = Velocitykm = kilometersmi = mileskph = kilometers per hourmph = miles per hour

Second Stage Ignitiont = 4.6 minutesAlt. = 115.6 km (71.9 mi)Vel. = 17400 kph (10810 mph)

Fairing Jettisont = 4.9 minutesAlt. = 129.6 km (80.7 mi)Vel. = 17620 kph (10950 mph)

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Illustration of theICESat launch.Graphic by DeborahMcLean and BoeingCorp.

Mission Operations And Calibration/ValidationOnce ICESat is on orbit, mission operations will be conducted by two organizations. The Earth ScienceData and Information System (ESDIS) Project at GSFC will provide space and ground network support.The University of Colorado’s Laboratory for Atmospheric and Space Physics (LASP) teamed with BallAerospace will provide mission operations and flight dynamics support. The GLAS data are recordedby the on-board Solid State Recorder and are played back during scheduled contacts via X-band down-link communications. The ICESat Science Investigator Processing System (I-SIPS) at GSFC will conductdata processing and generation of products with support from the Center for Space Research (CSR) atthe University of Texas, Austin. The National Snow and Ice Data Center (NSIDC), located at theUniversity of Colorado in Boulder, will archive and distribute ICESat data products to the scientificcommunity and other users. The interactions of these organizations are summarized in the chart below.

In the 60 days following launch, ICESat will undergo a series of tests to establish that all systems arefunctioning within specifications in the orbital environment. This commissioning phase will be followedby a period of intense activity to verify the performance of GLAS and all related systems. The objectiveof this intense period (calibration/validation or cal/val), is to help insure that geophysical interpreta-tions can be drawn from the data products. Cal/val includes evaluation of the GLAS measurementsagainst ground truth observations. A variety of instrumented and precisely mapped ground-truth siteswill be used, including dry lakebeds, landscapes with undulating surface topography, and the ocean, allof which will be periodically scanned.

Commandsand Telemetry

EPGSEOS Polar Ground System

EDOSEOS Data and Operations System

NSIDCNational Snow and Ice

Data Center

StandardProducts

Level 0Products

GPS andPrecision Rateand AttitudePacket Data

I-SIPSICESat Science Investigator-Led

Processing System

UTUniversityof Texas

SFCsScience

ComputingFacilities

OrbitalTrack.Info.System

CentralStd.Auto.FileSystem

MOCMission Operations Center

MMFDFMulti-Mission

Flight DynamicsFacility

Acquisition ofData Products

Science Data Playback

Schedules PlaybackH/K

Commands,Real-Time H/K

Real-TimeH/K

GLAS OperationsRequests

GLAS StatusLevel 0

Level 1 Products

Science Plan

Orbit and Attitude Data

Status

ESDIS

Level 1 POD and PADStandardProducts

ISFInstrument

SupportFacility

GroundStations

ICESat

Science

Level 0Products

H/K = Housekeeping

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Schematic diagram of ICESat mission operations. Graphic by Deborah McLean.

Acronyms

ReferencesICESat’s Laser Measurements of Polar Ice, Atmosphere, Ocean, and Land, H. J. Zwally, B. Schutz, W. Abdalati, J.Abshire, C. Bentley, A. Brenner, J. Bufton, J. Dezio, D. Hancock, D. Harding, T. Herring, B. Minster, K. Quinn, S.Palm, J. Spinhirne, and R. Thomas, Journal of Geodynamics, 34, 3-4, pp. 405-445, 2002.

Enhanced Geolocation of Spaceborne Laser Altimeter Surface Returns: Parameter Calibration from theSimultaneous Reduction of Altimeter Range and Navigation Tracking Data, S.B. Luthcke, C.C. Carabajal, and D.D.Rowlands, Journal of Geodynamics, 34, 3-4, pp. 447-475, 2002.

PARCA: Mass Balance of the Greenland Ice Sheet, Special Section of Journal of Geophysical Research -Atmospheres, 106, D24, pp. 33,689-34,058, 2001.

Laser Altimeter Canopy Height Profiles: Methods and Validation for Deciduous, Broadleaf Forests, D.J.Harding,M.A. Lefsky, G.G. Parker, and J.B. Blair, Rem. Sens. Environment, 76, 3, pp. 283-297, 2001.

System to Attain Accurate Pointing Knowledge of the Geoscience Laser Altimeter, J.M. Sirota, P. Millar, E. Ketchum,B.E. Schutz, and S. Bae, pp. 39-48 in AAS 01-003, Guidance and Control 2001, Advances in the AstronauticalSciences, R. D.Culp and C. N. Schira (eds.), 107, 2001.

Greenland Ice Sheet: High-Elevation Balance and Peripheral Thinning, W. Krabill, W. Abdalati, E. Frederick, S.Manizade, C. Martin, J. Sonntag, R. Swift, R. Thomas, W. Wright, and J. Yungel, Science, 289, pp. 428-430, 2000.

A Method of Combining ICESat and GRACE Satellite Data to Constrain Antarctic Mass Balance, J. Wahr, D.Wingham, and C.R. Bentley Journal of Geophysical Research 105, B7, pp. 16,279-16,294, 2000.

Geoscience Laser Altimeter System (GLAS), Algorithm Theoretical Basis Documents, (Version 3.0), A. C. Brenner, H.J. Zwally, C. R. Bentley, B. M. Csathó, D. J. Harding, M. A. Hofton, J.-B. Minster, L. Roberts, J. L. Saba, R. H. Thomas,and D. Yi, NASA Technical paper, 306p., 1999.

Monitoring Ice Sheet Behavior from Space, R. Bindschadler, Reviews of Geophysics 36, 1, pp. 79-104, 1998

Observations of the Earth's Topography from the Shuttle Laser Altimeter (SLA): Laser-Pulse Echo-RecoveryMeasurements of Terrestrial Surfaces, J. Garvin, J. Bufton, J. Blair, D. Harding, S. Luthcke, J. Frawley, and D.Rowlands, Physics and Chemistry of the Earth, 23, pp. 1053-1068, 1998.

Space Based Atmospheric Measurements by GLAS, J.D. Spinhirne and S.P. Palm, pp. 213-217, in Advances inAtmospheric Remote Sensing with Lidar, A. Ansmann (ed.), Springer, Berlin, 1996

More information on the ICESat mission can be found at: icesat.gsfc.nasa.gov as well as links to manyother supporting sites related to this and other EOS satellite missions.

ATBD Algorithm Theoretical Basis DocumentBall Ball Aerospace & Technologies Corp.CHIPSat Cosmic Hot Interstellar Plasma SpectrometerCSR Center for Space Research University of Texas, AustinEOS Earth Observing SystemEOSDIS EOS Data and Information SystemEPGS EOS Polar Ground System (Alaska and Svalbard)ESDIS Earth Science Data and Information SystemFOV Field Of ViewGLAS Geoscience Laser Altimeter SystemGPS Global Positioning SystemGRACE Gravity Recovery and Climate ExperimentGSFC Goddard Space Flight CenterICESat Ice, Cloud, and land Elevation SatelliteI-SIPS ICESat Science Investigator-led Processing SystemIST Instrument Star Tracker

JPL Jet Propulsion LaboratoryKSC Kennedy Space CenterLASP Laboratory for Atmospheric and Space Physics University of Colorado, BoulderLEO Low Earth Orbitlidar LIght Detection And RangingLRS Laser Reference SystemLSM Laser Select MechanismNASA National Aeronautics and Space AdministrationND:YAG Neodymium:Yttrium Aluminum GarnetNSIDC National Snow and Ice Data CenterPAD Precision Attitude DeterminationPOD Precision Orbit DeterminationSAR Synthetic Aperture RadarSCF Science Computing FacilitySLR Satellite Laser RangingSPCM Single Photon Counting Module

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Acknowledgements

ICESat ScientistsProject Scientist: Jay Zwally - NASA/GSFCDeputy Project Scientist: Christopher Shuman - NASA/GSFCProgram Scientist: Waleed Abdalati - NASA Headquarters

GLAS Science TeamTeam Leader: Bob Schutz - University of Texas-AustinTeam Members: Charles Bentley - University of Wisconsin-Madison

Jack Bufton* - NASA/GSFCDavid Harding (ex officio) - NASA/GSFCThomas Herring - Massachusetts Institute of TechnologyJean-Bernard Minster - Scripps Institution of OceanographyJames Spinhirne - NASA/GSFCRobert Thomas - EG&G CorporationJay Zwally - NASA/GSFC

GLAS Instrument TeamInstrument Scientist: Jim Abshire - NASA/GSFCInstrument Manager: Ron Follas - NASA/GSFCInstrument System Engineer: Eleanor Ketchum - NASA/GSFC

Lead Engineers:Laser Lead: Robert Afzal* - NASA/GSFC, Joe Dallas* - SSAISRS Lead: Pamela Millar - NASA/GSFC, Marcos Sirota - SigmaOptical Lead: Marzouk Marzouk - Sigma, Luis Ramos-Izquierdo - OSCLidar Lead: Michael Krainak - NASA/GSFCDetector Lead: Xiaoli Sun - NASA/GSFCElectrical Lead: Gregory Henegar, Steve Meyer - NASA/GSFCFlight Software Lead: Manuel Maldonado, Jan McGarry - NASA/GSFCMechanical Lead: Cheryl Salerno, Gordon Casto - NASA/GSFCThermal Lead: Eric Grob, Charles Baker, Walter

Ancarrow* - NASA/GSFCIntegration and Test Lead: Tom Feild - NASA/GSFC, Juli Landers - OSCBench Check Equip. Lead: Haris Riris - Sigma

Project Managers Joe Dezio, Jim Watzin, - NASA/GSFCDeputy Project Manager Linda Greenslade, Greg Smith - NASA/GSFCObservatory Manager Bill Anselm - NASA/GSFCMission Systems Engineer Tim Trenkle - NASA/GSFC

Brochure PreparationBrochure Writers: Jay Zwally and Christopher Shuman - NASA/GSFCBrochure Editors: Jim Abshire, Bill Anselm, Bob Schutz, James Spinhirne, Michael King,

Claire Parkinson, David Harding, Charlotte Griner, David Herring, Jack Saba, Anita Brenner

Brochure Design: Winnie Humberson - SSAICover Graphic Robert Simmon - SSAIICESat and GLAS Graphics Jason Budinoff - NASA/GSFCICESat Logo Design: Hailey King - Knott Laboratory, Inc.

NASA/GSFC NASA Goddard Space Flight CenterOSC Orbital Sciences CorporationSSAI Science Systems Applications, Inc.Sigma Sigma Research and Engineering Corporation* Changed affiliation

Science Measurement Requirements:• Measure ice sheet elevations to ≤ 1.5 cm/yr over 100x100 km areas• Measure all radiatively significant clouds and aerosols globally• Measure global land topography• Three year operational life (5 year goal)

Specifications: Surface1 AtmosphereWavelengths 1064 nm 532 nmLaser Pulse Energy 74 mJ 30 mJLaser Pulse Rate 40 Hz 40 HzLaser Pulse Width 5 nsec 5 nsecTelescope Diameter 1.0 m 1.0 mReceiver FOV 0.5 mrad 0.16 mradReceiver Optical Bandwidth 0.8 nm 0.03 nmDetector Quantum Efficiency 30% 60%Detection Scheme Analog Photon Counting Vertical Sampling Resolution 0.15 m 75 mSurface Ranging Accuracy (single pulse) 5 cmLaser Pulse Pointing Knowledge < 2 arcsec

1 Also measures backscatter from optically thick clouds

Measurement Approach:GLAS has 3 lasers, a single laser operates at any given time

Each Nd:YAG laser has 3 stages and emits pulses at 1064 and 532 nmcontinuously at 40 Hz

Precison orbital and pointing knowledge will be obtained via redundant GPS units, a gyro system, a laser reference sensor, and instrument-mounted star trackers, and ground-based laser ranging

Thermal control by heat pipes and radiators supplemented by heaters

Measurement Characteristics:GLAS operates at 40 pulses per second (Hz); ground track is ~70 m "spots"separated along-track by ~170 m; cross-track resolution is a function of the 183-day ground-track repeat cycle (15-km spacing at equator and 2.5km spacing at 80°latitude); orbit inclination is 94°

Altimetry measurements are determined from the round-trip travel time ofthe 1064 nm pulse

Cloud and aerosol data are extracted from the 532 nm pulse signal withoptically thick cloud tops extracted from the 1064 nm pulse signal

Post-processed laser pointing knowledge (1σ) will be ~2 arcsec

Post-processed position requirements: radial orbit for ice sheet to < 5 cm and along-track/cross-track position to < 20 cm

The ICESat spacecraft can control GLAS to point within 30 arcsec roll, 30arcsec pitch, and 1° yaw (3σ)

Physical CharacteristicsMass: 300 kg

Power: 330 W average

Data rate: ~450 kbps

Physical size: telescope diameter is 100 cm, instrument height is ~175 cm

GEOSCIENCE LASER ALTIMETER SYSTEM (GLAS)

Introduction To IceIce exists in the natural environment in many forms. The figure below illustrates the Earth’s dynamic ice features.At high elevations and/or high latitudes, snow that falls to the ground can gradually build up to form thick consol-idated ice masses called glaciers. Glaciers flow downhill under the force of gravity and can extend into areas thatare too warm to support year-round snow cover. The snow line, called the equilibrium line on a glacier or ice sheet,separates the ice areas that melt on the surface and become snow free in summer (net ablation zone) from the iceareas that remain snow covered during the entire year (net accumulation zone). Snow near the surface of a glacier that is gradually being compressed into solid ice is called firn.

Ice sheets, which are the largest forms of glaciers in the world, cover much of Greenland and Antarctica. Smaller icecaps are located in Iceland, Canada, Alaska, Patagonia, and mountainous regions of central Asia. These types oflarge ice mass have smaller outlet glaciers or ice streams near their margins. Mountain glaciers, smaller than icesheets or ice caps, flow from high mountain areas and are present on all continents except Australia. In some placeswhere the ice sheets reach the ocean, large floating ice shelves or floating glacier tongues are formed. Icebergs arefloating ice masses that have broken away from ice shelves, glacier tongues, or directly from the grounded ice sheetin some locations. Sea ice, which is produced when saline ocean water is cooled below its freezing temperature ofapproximately -2°C or 29°F, extends on a seasonal basis over great areas of the ocean.

Sea ice and icebergs are both carried by winds and currents into warmer waters. Melt water from sea ice, iceshelves, ice tongues, and icebergs does not contribute to sea level rise, because these ice masses already displace anequivalent amount of sea water. However, sea level rise is caused by the flow of grounded glacial ice into the oceanand by surface or subsurface melt water discharged from the glacier, if the sum of those amounts exceeds theamount of ice accumulated from snowfall on the glacier or ice sheet.

Illustration of ice in the natural environment. Graphic courtesy of Christopher Shuman, Claire Parkinson, Dorothy Hall, RobertBindschadler, and Deborah McLean.